Molality Calculator

Molality is the concentration of a solution measured in moles of solute per kilogram of solvent, written mol/kg or with the symbol m (some texts use b).

Molality equals the moles of solute divided by the mass of solvent in kilograms, b = n / m. The solvent mass is used, not the total solution mass, which is what the tool computes.

Molality uses the mass of solvent in kilograms, while molarity uses the volume of solution in litres. Because mass does not expand or contract with heat, molality stays fixed as temperature changes, whereas molarity drifts as the solution's volume shifts.

Divide the mass in grams by the molar mass in grams per mole. Switch to the Mass + molar mass option and the tool does this for you, then shows the derived moles alongside the molality.

Divide the grams by 1000. For example 250 g of water is 0.25 kg, and 750 g is 0.75 kg. Enter that kilogram value in the Mass of solvent field.

Molality and the derived moles are rounded to nine decimal places, so figures that do not divide evenly stay accurate. For instance 90 g of glucose (molar mass 180.156 g/mol) works out to about 0.499567 mol, which in 0.5 kg of water is about 0.999134 mol/kg.

The mass of solvent must be greater than zero, since dividing by zero is undefined, and neither the moles nor the mass can be negative. Molar mass must also be above zero. When an entry breaks one of these rules the tool shows a short error instead of a number.

For dilute water solutions the two numbers are close because a litre of water weighs about a kilogram. In concentrated or non-aqueous solutions they diverge, and for solvents denser than water molality often comes out larger than molarity.

Yes. The calculation runs entirely in your browser, so the masses and molar masses you enter never leave your device.